1. Field of the Invention
This invention relates generally to a thermal shutdown circuit and, more particularly to a monolithically integrated thermal shut-down circuit for power BiMOS processes.
2. Background Art
Many applications, e.g., automotive electronics, exist wherein an integrated circuit die is subjected to high temperatures due to on-chip power dissipation and high ambient temperatures. These integrated circuits may comprise a vertical power structure having a P-type substrate and an N-type epitaxial layer overlying the substrate. P-type regions are diffused into the epitaxial layer. PNP vertical transistors may be formed between a P-region and the substrate. Lateral PNP transistors may be formed using the P-regions and the epitaxial layer. This structure is well understood by those skilled in the art.
The large currents drawn through these vertical power transistors, especially during fault conditions, e.g., shorted output, can cause the integrated circuit die temperature to rise excessively. A typical method of sensing overheating of an integrated circuit die and for providing a switch function to shut down the heat generating circuitry is to compare the base-emitter voltage, V.sub.BE, of a bipolar transistor having a negative temperature coefficient with a circuit generated .DELTA.V.sub.BE voltage having a positive temperature coefficient.
This comparison may be accomplished by several different methods using bipolar transistors. One method comprises a bandgap voltage cell which generates a voltage proportional to .DELTA.V.sub.BE and compares it with V.sub.BE for providing a shut-down signal. The cell comprises first and second PNP transistors having collectors coupled to ground by first and second resistors, respectively, with the resistors having identical values. The collectors are also connected to the inverting input and noninverting input, respectively, of an amplifier. The output of the amplifier is connected to the bases of both transistors. The emitter of the first transistor is coupled to a node by a third resistor and the emitter of the second transistor is connected directly to the node. A fourth resistor is coupled between the node and a supply voltage. The first transistor has its emitter scaled to some multiple value greater than the emitter of the second transistor. In a manner known to those skilled in the art, and described in detail hereinafter, a voltage across the fourth resistor is proportional to the .DELTA.V.sub.BE between the first and second transistors and can be used for comparison to a V.sub.BE to accomplish a thermal shutdown function. The current through the fourth resistor however, when the cell is used in vertical output power transistor circuits, is subject to unknown quantities that causes the voltage across the fourth resistor to be an unkown quantity.
Any lateral PNP transistor, as used for the first and second transistor in the bandgap cell described above, can have a vertical current component collected from the P-type substrate. This occurs when the substrate is switched to a voltage level substantially equal to V.sub.CC supplied to the P-type regions serving as emitters of the PNP transistors. This vertical current is uncontrollable and unknown in value, substantially reducing the confidence level in predicting the voltage across the fourth resistor. Furthermore, if the .DELTA.V.sub.BE across the fourth resistor is compared to some .DELTA.V.sub.BE, then the bases of the first and second transistors must be two V.sub.BE voltages below V.sub.CC which cannot occur when the substrate is switched to V.sub.CC and the PN diode from the substrate to any N-type epitaxial region constrains all PNP bases to be only one V.sub.BE below V.sub.CC. Additionally, if the substrate is not switched to V.sub.CC, an unpredictable current exists due to a vertical component from the lateral emitters which is collected by the substrate in the absence of a buried layer.
Thus, a need exists for an improved thermal shut-down circuit that compensates for any vertical current component supplied to or from the substrate.